1. Field of the Invention
This invention relates to analog-to-digital converters and more particularly to analog-to-digital converters for the simultaneous converting of a plurality of analog signals into the corresponding digital values. More specifically, the analog-to-digital converter comprises a control means for controlling a ramp generator and digital counter such that the ramp generator and digital counter are maintained in a constant operational relationship.
2. Description of the Prior Art
It is common to provide systems which will produce digital readouts of the levels of analog signals deriving from multiple input channels on a multiplex basis. Typically, the several channels are sequentially sampled in some predetermined order and the samples apply to a single output channel including a suitable amplifier and analog-to-digital converter. When the analog input signals are of a low level type and slowly varying times, special precautions are necessary to provide an accurate readout and to prevent any deleterious loading effects on input signals by the measuring apparatus. In such cases the measuring apparatus must be of relatively high input impedance to provide the necessary isolation between input and output and must be extremely stable and of high sensitivity. Moreover, the amplifier must be responsive in rapid fashion to the voltage level differences which are present at the input terminals as each channel is sequentially connected to the single output channel. The prior art of multiple channel analog-to-digital converters is exemplified by U.S. Pat. Nos. 3,411,153; 3,448,446; 3,564,538; 3,455,397 and 3,742,489.
Conventional counting type analog-to-digital converters effect the conversion by counting the number of digital counts that occur during a period of time that is defined by the analog signal being converted. One specific type of counting analog-to-digital converter is that of the dual slope type. The prior art of analog-to-digital converter type is exemplified by U.S. Pat. Nos. 3,349,391; 3,624,638; 3,699,318; 3,728,625; and 3,747,089.